The present invention relates to a pressure control valve, and, more particularly, to an electromagnetic pressure control valve capable of controlling fluid pressure in response to an electric signal.
Pressure control valves are represented by a diaphragm type pressure control valve wherein the surface of the diaphragm receives feedback pressure and a spool type pressure control valve wherein the end surface of the spool which receives feedback pressure. The present invention relates to an electromagnetic pressure-control valve of the spool type.
The spool type electromagnetic pressure-control valve is usually designed in such a manner that a load caused by the electromagnet, a spring load, and an output pressure feedback load to act on a spool valve which slides in a valve sleeve having a supply port, an output port, and an exhaust port. Its structure is arranged such that the output is controlled to a level which corresponds to the electric signal input to the electromagnet portion by arranging the balance among the above-described loads (see Japanese Utility Model Laid-Open No. 60-52509).
Such a conventional spool type electromagnetic pressure-control valve is provided with a mechanism for adjusting the associated balancing spring. This mechanism allows the current vs. the hydraulic pressure characteristic of the control valve which is determined by the relationship between current supplied to the solenoid and the hydraulic pressure (output pressure) to be adjusted in parallel displacement, as shown in FIG. 1.
With the above-described adjustment, however, although the parallel displacement of the characteristic curve is possible, the gradient of the current vs. the hydraulic pressure characteristic curve cannot be adjusted.
This will be explained in detail with reference to FIG. 2.
The conventional practice is to adjust the load on the balancing spring in such a manner that the hydraulic pressure corresponding to the current value i.sub.b supplied to the solenoid falls within an allowable range of a given standard, i.e., the range between the broken lines shown in FIG. 2. However, when the control valve possesses a characteristic with a small gradient, such as the characteristic A, even if an adjustment point is set at the hydraulic pressure P.sub.1 which corresponds to the current value i.sub.b at the lower limit of the allowable range of the standard, the hydraulic pressure P.sub.2 outside the range corresponds to the current value i.sub.a when this current is supplied to the solenoid. Therefore, such control valves fail to meet the given standard. Similarly, when the control valve possesses a characteristic with a steep gradient, such as the characteristic B, even if an adjustment point is set at the hydraulic pressure P.sub.3 which corresponds to the current value i.sub.b at the upper limit of the allowable range, the hydraulic pressure corresponding to the current value i.sub.a would be outside the allowable range.
In view of the above-described circumstances, there is the risk that spool type electromagnetic pressure-control valves unable to meet standard may be produced in large numbers, particularly when they are mass-produced. This may lead to an increase in the proportion in which defective products are produced.
In order to avoid an increase in the proportion of defective products produced, it is necessary to restrict variations in the gradient of the current vs. the attractive force characteristic of the solenoid portion. This has hitherto called for improvement in the level of dimensional precision of various component parts, and has led to increase in production cost.